Transformers used in the Network Systems are unique in that they must be able to supply enough power to the system to keep the system at full load capability even if one of the transformers fail. When a transformer does fail it is imperative to get the transformer off the system as fast as possible before the fault in the transformer draws enough energy from the Network System to create great physical damage to the transformer and the environment surrounding the transformer.
Past practice has been to put secondary breakers on the transformers so that the transformers can be tripped open when a fault occurs on the secondary side of the circuit. These breakers have not been reliable in their fault sensing response or operation. Also, in some instances the fault occurs between the secondary breaker and the transformer in which case the secondary breaker is of no help.
Until recently, there has been no individual protection provided on the primary side of these transformers. However, because of recent catastrophic failures on the primary side of the transformer resulting in much property damage and in some instances personal injury, efforts are now being made to provide protection on the primary side of the transformer. High voltage devices such as circuit breakers, switches and electronically operated fuses have become available and are being applied in the field. These devices generally include energized electronic sensing circuits which are quite complicated requiring sophisticated electronics. The reliability of these circuits is questionable since they must be capable of operating over the 20 to 30 year life of the transformer. Since these devices are now intended to be the last resort of protection in taking a failing transformer off the Network System, before the transformer violently fails, a design having relatively high reliability and simplicity is required.
Full range fuses cannot be used since they can melt open when a transformer has to pick up more than its rated load which can occur when another transformer has failed and is taken off the system. Backup current limiting fuses can be used only if combined with a low current fault clearing system. Low current fault clearing systems must be capable of responding not only to fault currents that are lower than the melting current of the back up fuse but also to temperature and/or pressure sensors installed in the transformer vault and/or on the transformer. These sensors respond to high internal pressures and to arcing or fires inside the transformer vault. These sensors provide a trip signal that clears the transformer off the system.